1. Field of the Invention
Embodiments of the invention generally relate to an optical evanescent sensor utilizing a waveguide with an annular core.
2. Description of the Related Art
Various applications utilize optical sensors to measure parameters such as temperatures, pressures, biological compounds and chemicals in a particular environment. For example, uses of optical sensors include industrial sensing, research, medical analysis, and oil/gas recovery operations such as those occurring in a wellbore. These optical sensors can provide enhanced sensitivity, geometrical flexibility, miniaturization, durability in harsh environments with high temperatures and/or pressures, immunity from electromagnetic interference and multiplexing capabilities.
Some optical sensors function as evanescent sensors based on detection of changes in light propagating through an optical waveguide due to the optical mode penetrating evanescently into a surrounding media sensitive to a parameter being sensed. Techniques for producing evanescent sensors can rely on etching or polishing one side of an optical waveguide to produce a “D” shaped sensing element that brings the propagating mode along an interaction area in close proximity to the flat side such that a significant evanescent field is produced. However, long and complicated processes required to produce these sensing elements make such devices expensive. Further, the polishing or etching techniques used to produce the sensing element impose practical limits on a length of the interaction area that can be achieved since, for example, side-polished fiber portions are generally only a few millimeters long.
Therefore, there exists a need for an improved evanescent optical sensor that is easy to manufacture. A further need exists for a simple, flexible, monolithic, highly sensitive optical sensor element, which can be coupled to standard single mode optical fiber.